It becomes necessary, therefore, to find some other explanation of the low blood pressure which is the essential feature of shock. Of especial value in this connexion are the investigations by Keith, already mentioned, into the changes in blood volume found in soldiers suffering from shock and hæmorrhage. In very few of these cases were the symptoms due to shock alone, but usually the loss of blood volume was much greater than could be accounted for by the amount of hæmorrhage which had taken place. Here, therefore, was evidence strongly suggesting that the symptoms of shock are due to actual loss of circulating fluid, and the problem now resolved itself into a search for this fluid which has ceased to be part of the effective blood volume. Enough has already been said to show that there is no evidence that the larger vessels, whether arteries or veins, are acting as reservoirs in which the blood is stagnating. It therefore only remains to consider whether the capillary system is capable, under abnormal conditions, of holding so large a proportion of the blood as has been shown by Keith to have left the circulation. For a discussion of this problem the reader may be referred to W. B. Cannon’s summary of the arguments (45), from which it becomes clear that the capillary system may be regarded as a potential reservoir large enough to contain the lost blood in shock. The question is, however, further complicated by the fact that the capillary blood in shock differs from the circulating blood in containing an abnormal concentration of corpuscles. Extensive observations made on wounded soldiers have shown that the number of red blood cells may rise even to 8,000,000 per cmm. in the capillary blood, while the number in the venous blood remains at 5,500,000 or less. This concentration of the red cells is gradual and progressive, and will by itself account for a large part of the loss of volume, since normally the bulk of the blood is made up of corpuscles and plasma in approximately equal parts. The stagnation is, moreover, accentuated by the increased viscosity of the blood resulting from the concentration, and by the chilling of the surface of the body, which is always a feature of the state of shock. A vicious circle is thus established, and the symptoms of shock become severe as the capillary stagnation becomes more pronounced.